The C++17 standard introduces "template deduction guides". I gather they're something to do with the new template argument deduction for constructors introduced in this version of the standard, but I haven't yet seen a simple, FAQ-style explanation of what they are and what they're for.

In particular, I'd be interested to know whether any deduction guides are actually provided by the C++17 STL (e.g. for std::pair or std::tuple). What's the complete list of "deducible" standard template types as of C++17?
– QuuxplusoneFeb 21 '17 at 1:42

The compiler needs to figure out what vector<T>'s T type will be. We know what the answer is; T should be typename std::iterator_traits<Iterator>::value_type. But how do we tell the compiler without having to type vector<typename std::iterator_traits<Iterator>::value_type>?

This tells the compiler that, when you call a vector constructor matching that pattern, it will deduce the vector specialization using the code on the right of ->.

You need guides when the deduction of the type from the arguments is not based on the type of one of those arguments. Initializing a vector from an initializer_list explicitly uses the vector's T, so it doesn't need a guide.

The left side doesn't necessarily specify a constructor. The way it works is that, if you use template constructor deduction on a type, it matches the arguments you pass against all deduction guides (actual constructors of the primary template provide implicit guides). If there is a match, it uses that to determine which template arguments to provide to the type.

But once that deduction is done, once the compiler figures out the template parameters for the type, initialization for the object of that type proceeds as if none of that happened. That is, the deduction guide selected does not have to match the constructor selected.

This also means that you can use guides with aggregates and aggregate initialization:

@T.C. … unless the right thing is making a vector of iterators. And std::string{32,'*'}[0] == ' ' (for ASCII). But this has all been true since C++11.
– Arne VogelOct 20 '17 at 14:16

what happens with the allocator vector parameter ? what would happend if the allocator vector parameter wouldn't have a default argument? (you cannot deduce it from InputIterator)
– gnzlbgOct 27 '17 at 16:19

1

@NicolBolas: I see. It's not clear to me that the question is about explicit deduction guides at all... I think it's helpful if you just include what you literally wrote in this comment.
– MehrdadJul 13 '18 at 13:35